Proposals have been made in the past to utilize the high migration velocity of an arc to rapidly insert resistive runner rails into a circuit exposed to short-circuit currents. British Pat. Nos. 1 499 486 and 1 568 766 describe designs of this kind, in which the runner rails consist of straight or helically formed stiff bars.
In order to achieve an efficient current limitation upon a short-circuit in an a.c. circuit, with designs operating according to the above-mentioned principle, a considerable resistance must be inserted into the circuit even during the first milliseconds of the short-circuit period. Since the migration velocity of the arc admittedly is high but limited (500-1000 m/s), this rapid resistance insertion is only possible if the runner rails have a sufficiently high resistance per unit of length. At the same time the rails must have a high energy-absorption capacity, since no essential thermal dissipation by cooling is possible because of the rapid process. The designs described in the above-mentioned patent specifications do not fulfil the demand which - in these respects - are placed on current limiters for intermediate and high voltage.
Proposals have also been made to design a current limiter with two meander-shaped resistors made of insulated metallic tape and located opposite to each other, said resistors being inserted into the circuit with the aid of a current collector, being displaceable in the gap between the resistors (Swedish Pat. No. 192 481). With such a design it is difficult to achieve a sufficiently rapid insertion of the resistors in order to obtain an efficient current limitation.